A large force is needed to rotate the steering wheel of a vehicle for changing the direction of the tires when the vehicle is at rest or moving at a lower velocity. Particularly, front-engine front-drive automobiles which have recently gained popularity increasingly require greater force to turn the steering wheel, because large weight bears on the front portion of this kind of vehicle.
It is known that a power steering system amplifies the torque applied to the steering wheel by the driver. This system generates a driving force according to the torque applied by the driver and then transmits the force to the steering system. Most of the power steering systems now put into practical use are hydraulic in structure. Specifically, such a power steering system is equipped with a control valve, a hydraulic cylinder, etc. and produces an auxiliary torque by moving oil according to the torque applied by the driver.
Unfortunately, the control valve, hydraulic cylinder, and other components occupy large space. Further, pipes and similar components for connecting those components cannot be bent with a curvature smaller than a given value in order to prevent an occurrence of a large pressure loss. In addition, hydraulic structure must have certain and reliable sealings to prevent leakage of oil and is difficult to treat when mounted in the associated mechanism. Hence, it is difficult to install a power steering system in a vehicle having a small free space like a front-engine front-drive automobile.
Recently, the number of female drivers is on the increase. Although it is desired for female drivers that a vehicle can be steered with a small torque, such a vehicle does not give a good steering feeling to men of great physical strength. Also, when the vehicle is running at a higher velocity, the driver will be given a too light steering feeling. If the driver is not accustomed to the vehicle, he or she might turn the handle more than needed. It is also desired for men drivers of muscle that the vehicle can be steered with a small torque when they are tired, for example.
Some of conventional power steering systems can change the servo ratio, i.e., the ratio of the output torque of the power steering system to the input torque applied by the driver, In response to the velocity of the vehicle. However, even in this kind of vehicles the servo ratio is held constant for the same velocity. Therefore, if the characteristic of the ratio is set for women, then the system may be inconvenient for men, and vice versa.